Tag: Sahul

In the post below I mentioned that the Malaysian and Philippine Negritos seem to be two very distinct populations. This was something I wanted to explore in more detail, so I naturally decided to poke around the Pan-Asian SNP data set. The aims are made somewhat more difficult by the fact that there are only ~56,000 markers in the data set (as opposed to ~600,000 in the HGDP and more than 1 million in the HapMap). Additionally, the intersection with other data sets is small. For example, only ~20,000 SNPs with the HGDP. With all that in mind I hazarded that something is better than nothing. Relatives and HapMap populations were removed from the data set (thanks Zack). Additionally, I beefed up the South Asian populations with the Gujaratis from the HapMap,which had an intersection of ~32,000 SNPs. After a few test runs I decided to remove the Mlabri. They always shook out very early as a separate population from many others nearby, and, their genetic distances were very high. This tribe is only numbered in the hundreds, and I wouldn’t be surprised if they’ve been subjected to a lot of population bottlenecks, resulting in some very distinctive allele frequencies.

But before I move to the results, let’s back up for a moment. Who are the “Negritos”? As suggested by the term Negrito refers to a range of populations which are characterized by small size and African-like features (very dark skin and frizzy hair). In general their distribution is limited to Southeast Asia (there are suggestions that a Negrito population may only recently have gone extinct in Australia’s rainforests, but that’s speculative. On a more antique scale there are records which may be interpreted to suggest the existence of Negritos in Taiwan as late as 1900, and in southern China within the past 1,000 years). So you can bracket their distribution from the Andaman Islands to the Philippines, with isolated groups in the Malay peninsula. Negritos are presumed to be the original inhabitants of Southeast Asia before the arrival of rice farmers from the north. Like the Pygmies of Africa most of the Negritos speak languages whic hare known in other populations. Those of the Philippines speak Austronesian dialects. Interestingly those of Malaysia speak an Austro-Asiatic language, and so have affinities with many groups to their north linguistically, being surrounded by Austronesian speakers. Only the Andaman Islanders have a distinctive language, which makes sense seeing as how they have been relatively isolated from mainland Asian influences.

I ran ADMIXTURE from K = 4 to K = 12. K = 8 seemed the most informative to me (at higher K’s the major dynamic is that the Philippine Negritos start fragmenting into many distinct clusters). I’ve made a few cosmetic changes. With this East and Southeast Asia heavy data set there’s almost no difference between all the various Indian groups, so I amalgamated them together. I also did the same for related populations geographically adjacent which exhibited no genetic difference (e.g., Central and East Javanese).

“…the occupation of Australia/New Guinea is momentous in that it demanded watercraft and provides by far the earliest evidence of their use in history. Not until about 30,000 years later (13,000 years ago) is there strong evidence of watercraft anyway else in the world, from the Mediterranean.

Initially, archaeologists considered the possibility that the colonization of Australia/New Guinea was achieved accidentally by just a few people swept to sea while fishing on a raft near an Indonesian island. In an extreme scenario the first settlers are picture as having consisted of a single pregnant young woman carrying a male fetus. But believers in the fluke-colonization theory have been surprised by recent discovers that still other islands, lying to the east of New Guinea, were colonized soon after New Guinea itself, by around 35,000 years ago….”

The settlement of Australia is a breakthrough in the “human story.” Very soon after anatomically modern humans began to replace (and to some extent assimilate) other lineages of our genus in Eurasia we pushed beyond the previous outer limits of the domains of humankind. The ancestors of Australian Aboriginals swept past the Wallace Line, and quickly settled the Ice Age continent of Sahul, consisting of Australia and Papua New Guinea. The biogeography of Australia is well known. Aside from bats and some endemic rodents the continent was free of placental mammals before modern humans arrived.

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